Audited —·Last updated 26 Jul 2026·7 citations·Tier 1·0 uses

Unit Converter

Free unit converter for length, mass, volume, temperature, time, area, and speed. SI and US customary units with NIST-grade conversion factors.

Unit Converter

Quantity
The number you want to convert. Negative values are allowed only for temperature (everything else is a positive magnitude).
From unit
To unit
Converted value
160.9344
The input value expressed in the target unit, computed at full Decimal.js precision and rendered to the precision your context requires.

Background.

This unit converter handles the seven quantities a working calculator user actually needs — length, mass, volume, temperature, time, area, and speed — using the conversion factors published by the National Institute of Standards and Technology (NIST Special Publication 811) and the International Bureau of Weights and Measures (BIPM SI Brochure, 9th edition). Pick a quantity, type a value, choose the from-unit and the to-unit, and the calculator returns a result computed at arbitrary precision (decimal.js, twenty-significant-digit working precision) so a mile is exactly 1609.344 meters and a US gallon is exactly 3.785411784 liters — the defined values, not floating-point approximations that drift in the seventh decimal place.

The point of a unit converter in 2026 is not to do the arithmetic — every smartphone calculator can do that — but to do the arithmetic with the right factor. The right factor is the one defined by treaty (the BIPM redefined the SI base units in 2019 in terms of seven fixed natural constants, so the meter and the second are now fixed by physics, not by an artifact), recognized by the regulator who will inspect your scale (NIST Handbook 44 is the US legal-metrology reference; the international avoirdupois pound is defined as exactly 0.45359237 kilograms, set by international agreement in 1959 and codified in NIST SP 811), and traceable back to the primary source rather than copied from a Wikipedia table that copied another Wikipedia table.

The calculator is built around two distinct mathematical paths because units do not all behave the same way. For length, mass, volume, time, area, and speed, conversion is purely multiplicative: every unit is expressed as a multiple of a canonical base unit (the meter for length, the gram for mass, the liter for volume, the second for time, the square meter for area, the meter per second for speed), and converting from one unit to another is just multiplication by the from-factor and division by the to-factor.

Temperature is different, and the difference matters: the Celsius and Fahrenheit scales are affine, not linear, because each has a zero point that is offset from absolute zero. You cannot convert 20 °C to Fahrenheit by multiplying by a single factor — the conversion has both a scale (5/9 or 9/5) and a shift (the 32 in F = 1.8C + 32, or the 273.15 in K = C + 273.15). The calculator handles this by routing every temperature through the absolute Kelvin scale: it converts the input to Kelvin first, then from Kelvin to the target unit, which guarantees that round-tripping (C → F → C) returns the original value exactly and that no negative-zero or sign-mismatch bug can sneak in.

Below the widget you will find a section on what unit conversion actually is mathematically (a coordinate transform between two number lines that measure the same physical quantity), a walk-through of the factor approach with worked examples in each of the six linear categories, a separate walk-through of the Kelvin-anchor method for temperature with the algebra spelled out, a real-world example (100 miles per hour expressed in kilometers per hour, which is the calculator's headline test case and resolves to exactly 160.9344 km/h because both factors are defined by treaty), and FAQs covering the questions users actually ask: why a US gallon is different from an imperial gallon (the US Customary gallon is 3.785411784 L, the UK imperial gallon is 4.54609 L, a 20 percent gap that has caused enough fuel-economy confusion that the EPA forced US window stickers to show liters per 100 kilometers as a secondary metric), why the calculator uses the Julian year (31,557,600 seconds, the astronomical reference, rather than the 365- or 365.25-day civil approximations), what a knot is (exactly 1.852 km/h, the nautical mile per hour, set by international agreement and used by every aviation and maritime authority on Earth), and when to use SI versus customary units in scientific writing (SI everywhere except where local regulation requires customary, with the customary value parenthesized; this is the ISO 80000 and NIST SP 811 guidance and the convention every peer-reviewed engineering journal enforces).

The goal is a converter you can trust for engineering work, scientific reporting, cooking translations from European recipes, road-trip planning across the US-Canada border, and the kind of casual sanity checks (how heavy is 50 stone in kilograms, how big is an acre in square meters) that get typed into a search box every day.

What is unit converter?

A unit converter changes the expression of a physical quantity from one unit to another while preserving the underlying quantity. The quantity itself — a length, a mass, a duration — is an invariant of the physical world; the unit is the yardstick you choose to measure it against, and any two yardsticks that measure the same quantity are related by a fixed conversion factor. Mathematically the conversion is a coordinate transform between two number lines. For most quantities the transform is linear and passes through the origin: zero meters is zero feet, zero kilograms is zero pounds, zero seconds is zero hours. That means the conversion is single-multiplicative — multiply by the from-unit's factor (its size in the canonical base unit) and divide by the to-unit's factor, or equivalently multiply by the ratio of the two factors. The base units the calculator uses internally are the SI coherent base or commonly accepted units: meter for length, gram for mass, liter for volume, second for time, square meter for area, meter per second for speed. The conversion factors come from NIST Special Publication 811 (the US legal reference) and the BIPM SI Brochure, 9th edition (the international reference). The exception is temperature. The Celsius scale is offset from absolute zero by 273.15 (zero Celsius is the freezing point of water, not the zero point of thermal energy), and the Fahrenheit scale is offset further and uses a different unit-of-scale (5/9 of a kelvin per degree Fahrenheit). The Celsius-to-Fahrenheit transform is therefore affine: F = (9/5) × C + 32. You cannot collapse it to a single multiplication. The calculator handles temperature by always routing through Kelvin — the absolute scale anchored at thermodynamic zero — which makes the two-step conversion symmetric and round-trip-safe: the algorithm converts the input to Kelvin first, then from Kelvin to the target unit, so the same code handles C-to-F, F-to-C, K-to-C, C-to-K, and so on, without special cases or sign-handling bugs.

How to use this calculator.

  1. Choose the quantity you are converting from the Quantity dropdown — length, mass, volume, temperature, time, area, or speed. This tells the calculator which factor table to use and which from/to-unit pairs are legal.
  2. Enter the numeric value in the Value field. For temperature, negative numbers are allowed (and meaningful — minus 40 °C equals minus 40 °F is the calculator's classic round-trip test). For everything else, use a positive magnitude.
  3. Pick the From unit. The dropdown shows the full list of supported units across all categories; the calculator validates that the unit you pick belongs to the quantity you selected and will reject (with a clear error) a mismatch like 'length: 5 kilograms'.
  4. Pick the To unit. Same dropdown, same validation rule — it must belong to the same quantity as the from-unit.
  5. Read the Converted value. The number is computed at twenty-significant-digit working precision and rendered to a precision appropriate for the context (six decimal places for most quantities, with trailing zeros suppressed). The Conversion field shows the human-readable expression — input value, conversion factors, result — so you can sanity-check the math by hand if you are using the result in published work.
  6. For repeated conversions in the same category (e.g. cooking translations across multiple ingredients), keep the Quantity setting fixed and just change Value and the unit dropdowns — the calculator does not require a full reset between conversions.

The formula.

There are two algorithms behind this calculator — one for the six linear quantities and one for temperature.

For length, mass, volume, time, area, and speed, each supported unit is stored as its size in the canonical base unit. Length is anchored in meters: a kilometer is 1000 m, an inch is 0.0254 m (NIST SP 811, exact by 1959 international yard-and-pound agreement), a mile is 1609.344 m (5280 × 0.3048 ft × m/ft, exact). Mass is anchored in grams: a kilogram is 1000 g, an avoirdupois pound is 453.59237 g (exact, NIST SP 811), a stone is 14 × 453.59237 = 6350.29318 g. Volume is anchored in liters and uses US customary by default per NIST: a US gallon is 3.785411784 L (exact by definition, 231 cubic inches), a US fluid ounce is 1/128 of a gallon. Time is anchored in seconds and uses the Julian year (31,557,600 s = 365.25 × 86400 s, the astronomical reference adopted by the IAU). Area is anchored in square meters: an international acre is 4046.8564224 m² (NIST SP 811), a square mile is 640 acres. Speed is anchored in meters per second: a knot is exactly 0.514444 m/s (nautical mile per hour, ICAO/IMO standard), 1 km/h is 1/3.6 m/s exactly.

Given a value V in the from-unit with factor F_from and a to-unit with factor F_to, the conversion is one multiplication and one division:

result = V × F_from / F_to

This works because V × F_from is the value expressed in the base unit (meters, grams, liters, seconds, m², m/s), and dividing by F_to re-expresses that base-unit value in the target unit. Internally the calculator uses decimal.js arithmetic with twenty significant digits of working precision so that defined factors like 0.0254 and 0.45359237 do not lose precision to binary floating-point rounding — the same calculation in raw JavaScript double-precision would drift in the fifteenth or sixteenth decimal place, which is harmless for cooking but unacceptable for engineering work.

For temperature the algorithm is two-step affine, anchored on Kelvin:

to-Kelvin: if from = C: K = C + 273.15 if from = F: K = (F − 32) × 5/9 + 273.15 if from = K: K = K (identity)

from-Kelvin: if to = C: C = K − 273.15 if to = F: F = (K − 273.15) × 9/5 + 32 if to = K: K = K (identity)

Why route through Kelvin? Because Kelvin is the absolute thermodynamic scale anchored at zero point of molecular kinetic energy, with the same unit-of-scale as Celsius (one kelvin equals one Celsius degree by definition since the 2019 SI redefinition fixed the kelvin via the Boltzmann constant). Using Kelvin as the canonical pivot makes every pairwise conversion symmetric: C-to-F is exactly the reverse of F-to-C, and the round trip C → K → F → K → C returns the original value exactly. The naive single-step formula F = (9/5)C + 32 is correct, but writing it that way for every pairwise direction (C→F, F→C, C→K, K→C, F→K, K→F) is six formulas with six opportunities for a sign or coefficient bug; the Kelvin-anchor approach is two functions, each handling one direction, and the engine composes them. The same Kelvin-anchor architecture is used by the NIST Chemistry WebBook, by every engineering thermodynamics textbook, and by the IUPAC Compendium of Chemical Terminology.

A worked example.

Example

Converting 100 mph to km/h is the classic North-America-to-Europe driving-speed conversion, and it is also the cleanest possible audit of the calculator's precision because both factors are defined by treaty rather than measured. The mile is exactly 1609.344 meters (5280 × 0.3048 m/ft, by the 1959 international yard-and-pound agreement and codified in NIST SP 811), the hour is exactly 3600 seconds (BIPM, definitional), so 1 mph is exactly 1609.344 / 3600 = 0.44704 m/s — the value the calculator's internal speed table uses. The kilometer per hour is exactly 1000 / 3600 = 1/3.6 m/s. Multiplying 100 mph by the from-factor: 100 × 0.44704 = 44.704 m/s, the speed expressed in the SI base unit for speed. Dividing by the to-factor (1/3.6): 44.704 / (1/3.6) = 44.704 × 3.6 = 160.9344. The result is exactly 160.9344 km/h — not 160.93, not 160.934, but 160.9344 to the last digit, because every factor in the chain is exact by definition. The same conversion shows up in published US highway data (Federal Highway Administration speed-limit reports) and on the dashboard of every car sold in Canada, the UK, Australia, and most of the rest of the world, where the speedometer's primary scale is km/h and the mph scale is a small inner ring for the small fraction of drivers who cross into the US. The Conversion output reads: 100 mph × 0.44704 ÷ 0.2777778 = 160.9344 km/h, which is the same computation the calculator's source code performs internally and the form in which you can transcribe the result into a lab notebook or a report and have a peer reviewer verify it without re-running the tool.

to Unitkmh
categoryspeed
value100
from Unitmph

Frequently asked questions.

What is the difference between SI units and US customary units, and which should I use?
SI (the International System of Units, defined by the BIPM and adopted by every country except the US, Liberia, and Myanmar as the legal system of measurement) is a coherent system built on seven base units — meter, kilogram, second, ampere, kelvin, mole, candela — with derived units like the newton and the joule defined as products and quotients of the base units. US customary (foot, pound, gallon, Fahrenheit) is a legacy system whose modern definitions are fixed in terms of SI: the international foot is exactly 0.3048 m, the avoirdupois pound is exactly 0.45359237 kg, the US gallon is exactly 3.785411784 L. For scientific work, engineering specifications, medical dosing, and any cross-border or peer-reviewed publication, use SI — it is the convention every engineering journal, every ISO standard, and the entire scientific literature has adopted, and it eliminates the customary-vs-imperial gallon ambiguity that has caused real engineering failures (most famously the 1983 Gimli Glider Air Canada incident, where a fuel-quantity miscalculation in pounds-vs-kilograms ran a Boeing 767 out of fuel at altitude). For US legal-metrology contexts — fuel pumps, retail scales, road signs, EPA fuel-economy ratings — use US customary because that is what the regulator requires. NIST SP 811's general guidance is SI everywhere with the customary value parenthesized when local regulation requires it: '100 km/h (62 mph)'.
Why does this calculator use the US gallon instead of the imperial gallon?
Because the US gallon is the legally recognized gallon in the United States and the calculator is operating under NIST SP 811 conventions. The US gallon is 3.785411784 L (exact by definition, 231 cubic inches per the 1707 Queen Anne wine gallon adopted into US law). The UK imperial gallon is 4.54609 L (exact by the 1985 UK Weights and Measures Act), about 20 percent larger. The gap matters enough that EPA fuel-economy stickers in the US show MPG calibrated on the US gallon, while UK and Caribbean fuel-economy figures published in MPG are calibrated on the imperial gallon — a UK 40-MPG figure is equivalent to about 33.3 US-MPG, and confusing the two has cost more than one road-trip driver a fueling station's worth of distance. If you need imperial volume conversions, convert the US-gallon result by multiplying by 1.20095 (1 imperial gallon = 1.20095 US gallons) — or use a calculator with an explicit UK-units mode. Most American volume cookware (cups, teaspoons, tablespoons, fluid ounces) is calibrated on the US gallon, which is why this calculator uses it as the default.
Why is temperature handled differently from the other quantities?
Because temperature is the only quantity in the calculator with an affine, not linear, relationship between its scales. Length, mass, volume, time, area, and speed all share an absolute zero — zero meters is zero feet is zero anything, the origin of the number line is the same for every unit. Temperature does not: zero degrees Celsius is the freezing point of water (273.15 K, well above thermodynamic zero), zero degrees Fahrenheit is the calibration point Daniel Fahrenheit chose in 1724 (255.37 K, the freezing point of an ammonium-chloride brine), and zero Kelvin is the actual thermodynamic floor where molecular kinetic energy goes to zero. To convert between two scales with different zero points you need both a multiplicative scale factor and an additive offset — F = (9/5)C + 32 has both — and you cannot collapse the conversion to a single multiplication the way you can for, say, miles to kilometers. The calculator handles this by routing every temperature through Kelvin: convert to Kelvin first, then from Kelvin to the target unit. The architecture guarantees that round-tripping is exact (C → K → F → K → C returns the input precisely) and that the same algorithm handles every pairwise direction without special-case code per pair. This is the same approach used by the NIST Chemistry WebBook and IUPAC's Compendium of Chemical Terminology.
Why does a year equal 31,557,600 seconds in the time category, and not 365 × 86,400?
Because the calculator uses the Julian year, the astronomical reference adopted by the International Astronomical Union in 1976 and used in essentially every scientific time-conversion table: 1 Julian year = 365.25 × 86,400 = 31,557,600 seconds exactly. The 0.25-day fractional component reflects the leap-year correction — three out of every four years are 365 days and one is 366, so the average is 365.25. Using a flat 365-day year (31,536,000 s) would understate the year by 6 hours and produce a 0.07 percent error in long-duration conversions; using a tropical year (the time between two consecutive vernal equinoxes, 365.24219 days) would be more accurate astronomically but is rarely used outside of calendar-reform research. NIST SP 811 explicitly cites the Julian year as the conventional astronomical year for unit-conversion purposes, and that is the figure the calculator uses. If you need civil-calendar arithmetic (e.g. 'how many days between two dates in the Gregorian calendar'), use the dedicated Age Calculator or Date Difference Calculator, which handle the irregular month lengths and leap-year rules explicitly.
How accurate are the conversion factors in this calculator?
Every factor in the calculator is either defined by international agreement or fixed to its full published precision by a recognized standards body. The international foot, yard, mile, and avoirdupois pound have been exact since the 1959 international yard-and-pound agreement (US foot = 0.3048 m exactly; pound = 0.45359237 kg exactly). The US gallon is exact by definition (3.785411784 L, derived from 231 cubic inches). The hectare and acre are exact (10,000 m² and 4046.8564224 m² respectively, per NIST SP 811). The Julian year is exact (365.25 × 86,400 s). The knot is exact (1.852 km/h or 0.514444 m/s, by ICAO/IMO standardization in 1929). The temperature offsets (273.15 K, 32 °F) are exact by definition since the 2019 SI redefinition fixed the kelvin via the Boltzmann constant. Working precision is 20 significant digits via decimal.js, which means rounding errors do not appear until far beyond the precision any user actually cares about (typically 6 to 10 significant digits). The factors and their sources match NIST SP 811 (2008 edition) and the BIPM SI Brochure, 9th edition (2019), and the calculator's behavior is unit-tested against the round-trip identity (X → Y → X returns X) and against published reference conversions to ensure no drift.
Why is the knot used for aircraft and ship speed instead of km/h or mph?
Because the knot — one nautical mile per hour, exactly 1.852 km/h or 0.514444 m/s — is anchored to the geometry of the Earth in a way that km/h and mph are not. One nautical mile is defined as 1852 meters (international, 1929 ICAO/IMO), which is approximately one minute of arc of latitude along any meridian; equivalently, 60 nautical miles is one degree of latitude. This means that at sea or in the air, where navigation is done in latitude/longitude and great-circle distances on a sphere, a speed in knots translates directly into degrees-per-hour of latitude crossed, which is the natural unit for celestial-navigation and dead-reckoning calculations. Every aviation chart, every nautical chart, every international NOTAM (Notice to Airmen), and every air-traffic-control voice transmission uses knots; converting to km/h or mph would add a multiplication step to every position fix without any operational benefit. The knot is one of the small number of non-SI units the SI Brochure explicitly accepts for use 'whose values must be obtained by experiment' or that are 'in widespread use, but whose use is generally restricted to particular subject areas'.
What is the difference between mass and weight, and which does this calculator measure?
The mass category in this calculator measures mass — the quantity of matter, an invariant property of an object that does not change with location. The kilogram, gram, pound (avoirdupois), and ounce as listed in the calculator are all units of mass in the SI / NIST SP 811 framework, even though in everyday English the word 'weight' is often used interchangeably. Strictly, weight is the gravitational force exerted on a mass and is measured in newtons (kg·m/s²); a 70 kg person 'weighs' about 686 N on Earth's surface but only about 113 N on the Moon, while their mass remains 70 kg in both places. The everyday confusion is harmless on Earth at sea level because g (9.80665 m/s², standard gravity by definition) is nearly constant, but it matters in aerospace, in laboratory metrology, and any time you are computing forces (a force calculator that asks for 'weight in kilograms' is technically asking for mass and applying g implicitly). If you need to convert force or compute weight as a force, use a dedicated force / Newton's-second-law calculator; this converter handles only the scalar mass conversion.
How do I convert L/100km to miles per gallon for fuel economy?
The relationship is reciprocal, not linear: MPG = 235.215 / (L/100km), and equivalently L/100km = 235.215 / MPG. The constant 235.215 comes from (1 mile per 1.609344 km) × (3.785411784 L per US gallon) × 100. A car rated at 7.0 L/100km gets 235.215 / 7.0 ≈ 33.6 US-MPG; a car rated at 30 MPG consumes 235.215 / 30 ≈ 7.84 L/100km. Because the relationship is reciprocal, equal-percentage improvements look very different on the two scales — which is the entire reason the EPA in 2013 added gallons-per-100-miles to US fuel-economy window stickers (the linear consumption metric tracks fuel savings proportionally, while MPG compresses the high end of the scale). For UK imperial-gallon MPG, the constant is 282.481 instead of 235.215 (because the UK gallon is 4.54609 L). This calculator does not currently include a dedicated fuel-economy mode — use the Fuel Cost Calculator for that workflow, which handles both US-MPG and metric L/100km natively and adds price and CO2 outputs.
Why does the calculator not include 'natural' units like light-years or atomic mass units?
Because the calculator is scoped to the seven everyday-use quantities a working calculator user converts most often — length, mass, volume, temperature, time, area, speed — at the human scale (millimeters to kilometers, milligrams to tonnes, milliliters to gallons, milliseconds to years). Specialty units like the light-year (9.461 × 10^15 m, astronomical-distance scale), the parsec (3.086 × 10^16 m), the atomic mass unit (1.66054 × 10^-27 kg), the electronvolt (1.602 × 10^-19 J, energy not length), and the angstrom (10^-10 m, crystallography) belong in domain-specific calculators where the surrounding context — astronomy, particle physics, materials science — informs which conversions are most useful and which secondary outputs (e.g. redshift, momentum) should appear alongside the raw number. Stuffing every unit into one general-purpose converter degrades the user experience for the 95 percent of conversions that stay at the human scale. If you need astronomical, atomic, or energy units, see the dedicated Wavelength/Frequency, Gravitational Force, or Kinetic Energy calculators — each has its own factor table calibrated to the precision that domain requires.
Can I trust the calculator for legally-binding measurements (contracts, customs declarations, scale certifications)?
For the arithmetic, yes — every factor in the table is sourced from NIST SP 811 (the US legal-metrology reference) or the BIPM SI Brochure, 9th edition (the international SI reference), and the working precision (twenty significant digits via decimal.js) far exceeds the precision required by any commercial-grade scale or fuel meter. For the legal-metrology context, no calculator is a substitute for a certified, traceable scale or meter calibrated by a NIST-traceable laboratory (or the equivalent national-measurement institute in your jurisdiction). Use the calculator to plan, verify, and sanity-check; use a certified instrument and a calibration certificate for the legally-binding measurement. If you are filing a customs declaration, a tax return that depends on a quantity expressed in customary units, or a contract clause referencing a measurement, the controlling authority for the conversion factor is whichever standards body the relevant statute or regulation cites — usually NIST in the US, the National Measurement Office in the UK, or the regional NMI in continental Europe — and the factors in this calculator agree with all of them to at least eight significant digits.

References& sources.

  1. [1]National Institute of Standards and Technology. NIST Special Publication 811, 2008 Edition — Guide for the Use of the International System of Units (SI). The authoritative US reference for SI usage, derived-unit definitions, and exact conversion factors between SI and US customary units (foot, pound, gallon, acre, mile). Every linear-conversion factor in this calculator traces to SP 811.
  2. [2]Bureau International des Poids et Mesures (BIPM). The International System of Units (SI Brochure), 9th edition, 2019. The treaty-level reference for the SI base units following the 2019 redefinition that fixed all seven base units via fundamental constants (Planck constant, Boltzmann constant, elementary charge, etc.). Source for the kelvin definition that underpins the calculator's temperature handling.
  3. [3]International Organization for Standardization. ISO 80000 series — Quantities and units (Parts 1–14). The international standard for the names, symbols, and definitions of quantities and units across science and engineering. Reviewed periodically; the calculator's symbol conventions (kg, m, s, K, mol) follow ISO 80000-1 (general principles).
  4. [4]National Institute of Standards and Technology. Handbook 44 (2024 edition) — Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices. The US legal-metrology reference adopted by every US state for commercial scales, fuel meters, and retail measuring devices. Source for the US gallon, US fluid ounce, and avoirdupois pound exact definitions.
  5. [5]International Astronomical Union. Resolution B2 (2012) — Re-definition of the Astronomical Unit of Length, and adoption of the Julian year (365.25 days × 86 400 s = 31 557 600 s) as the conventional astronomical year. Source for the calculator's year-to-second conversion factor in the time category.
  6. [6]International Union of Pure and Applied Chemistry (IUPAC). Compendium of Chemical Terminology, 'Gold Book', entry on Temperature (T). Confirms the kelvin as the SI base unit for temperature and the 273.15 K offset relating the Kelvin and Celsius scales; the conceptual basis for the calculator's Kelvin-anchored temperature algorithm.
  7. [7]International Civil Aviation Organization (ICAO). Annex 5 to the Convention on International Civil Aviation — Units of Measurement to be Used in Air and Ground Operations. Source for the international knot (exactly 1.852 km/h) and the nautical mile (exactly 1852 m) used in the calculator's speed category.
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